<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Document</title>

        <style>
            
            .box1 {
                border: 10px red solid;
            }

            .box2 {
                width: 100px;
                height: 100px;
                background-color: #bfa;

                float: left;
            }
            
            /* 方式一 */
            /* .box3 {
                clear: both;
            } */
            
            /* 方式2:选中元素最后面 */
            .box1::after {
                content: '';
                display: block;
                clear: both;
            }

        </style>


    </head>
    <body>
        <div class="box1">
            <div class="box2"></div>

            <!-- <div class="box3">
            </div> -->
        </div>
    </body>
</html>